1. Field of the Invention
The invention relates to a solenoid operated trip for releasing an electrical contactor, and in particular to an improved miniaturized contactor having a solenoid operated trip in lieu of a flux transfer shunt associated with a permanent magnet.
2. Prior Art
Circuit breakers and similar electrical contactors provide protection against faults in electrical power systems, such as overcurrent, ground fault and short circuit protection. In a typical contactor, a spring loaded plunger is arranged for movement between an extended trip position and a retracted normal position. In the extended position of the plunger, power connections to the load are opened. In the retracted position, the plunger is held by a permanent magnet, against the force of a spring urging the plunger to extend. A coil is associated with the permanent magnet. When a fault condition occurs, the coil produces a magnetic flux in opposition to the flux generated by the permanent magnet, thus releasing the force holding the plunger in the retracted position and allowing the spring to extend the plunger and open the power connections to the load. This arrangement is known as a flux shunt in that the coil shunts the permanent magnet. An example disclosed in U.S. Pat. No. 5,105,326--Shimp et al, also operates the flux shunt coil in the normal mode to assist the permanent magnet in holding the plunger in its retracted position (i.e., adding to the permanent magnet flux), thereby minimizing the incidence of nuisance trips.
Circuit breaker trip arrangements having a permanent magnet opposed by a spring rely on the balance of forces produced by the magnet and spring, respectively. The spring is set to a working compression or extension (e.g., compressed to a working height) that determines the force exerted. The extension or compression of the spring determines the force exerted, according to the spring constant of the spring. This force must be exceeded by the attractive force of the permanent magnet to hold the plunger in the normal retracted position. It is generally necessary to provide a means to adjust the spring arrangement initially to obtain the precise optimal spring force. Over time, the permanent magnet may become demagnetized, whereupon the spring is relatively more powerful than is optimal. Similarly, the spring may become relaxed such that the magnet is relatively more powerful. Application of releasing current to the flux shunt coil, which is an electromagnet, tends to demagnetize the permanent magnet due to the flux at a polarity opposite to that of the magnet. Whereas the balance of forces is sensitive to the extent of spring compression, and is affected by calibration and demagnetization of the magnet, nuisance trips may occur.
Additionally, permanent magnets have some inherent drawbacks. The coil-magnet arrangement is relatively large. The plunger-magnet assembly has a large mass, presenting a danger of tripping due to shock. The magnet also undesirably attracts magnetic debris. It would be advantageous to provide a function similar to a flux shunt arrangement that is not sensitive to calibration of the spring, is compact, and avoids the problems associated with permanent magnets.
Solenoid operated breaker mechanisms are known in some variations. U.S. Pat. No. 4,408,174--Seymour et al discloses a solenoid operable to release a plunger when the control voltage drops below the level necessary to withstand the force of a spring. U.S. Pat. No. 4,301,433--Castonguay et al discloses an example of a breaker mechanism having a holding solenoid that operates a toggle linkage when released.
The present invention employs a solenoid with a spring loaded actuator restrained by a latched lever forming a trigger for tripping the breaker, in a simple, compact and durable arrangement that is insensitive to spring constants, and has no permanent magnet.